The present invention relates to a catalyst comprising silica and an acid phase containing sulphuric acid and the compound HB(HSO.sub.4).sub.4, the silica having been impregnated by said acid phase. The invention also relates to the preparation and use of said catalyst in catalytic alkylation of isoparaffin (isobutane and/or isopentane) in the presence of at least one olefin containing 3 to 6 carbon atoms per molecule.
In order to supply internal combustion engines which have spark ignition, and particularly engines with a high compression ratio, it is particularly advantageous to have fuels with high octane ratings, that is to say consisting essentially of heavily branched paraffin hydrocarbons. The alkylation of isoparaffins (isobutane and/or isopentane) by at least one olefin containing 3 to 6 carbon atoms per molecule allows such products to be obtained. This reaction requires the use of very acidic catalysts, particularly for reducing parasitic reactions such as hydride abstraction from the olefin and of polymerization which provide slightly branched hydrocarbons with a low octane rating and unsaturated hydrocarbons, cracking reactions and dismutation reactions.
The existing processes for production of hydrocarbons by alkylation of isobutane by olefins generally use either sulphuric acid or hydrofluoric acid as the catalyst. In these processes the acid catalyst constitutes a liquid phase which is placed in contact with the liquid isobutane/olefin(s) mixture to form an emulsion. These processes are expensive and give rise to significant problems with regard to personal and environmental safety. In order to remedy these problems, different catalytic systems of sulphuric acid and hydrofluoric acid in liquid phase have been sought.
In order to catalyze the alkylation reactions of isoparaffins by olefins, it has already been proposed to develop acid catalysts from numerous acid solids of different types. Amongst the families of acid catalysts, molecular sieves, macroreticular resins, possibly associated with BF.sub.3, Lewis and/or Bronsted acids deposited on various inorganic supports, chlorous aluminiums, graphites interpenetrated with Lewis and/or Bronsted acids and anions deposited on oxide supports such as ZrO.sub.2 /SO.sub.4 can be cited. These solids lead to the production of branched isoparaffins but suffer from several major faults, among which the use of often very high isobutane/olefin molar ratios to limit the magnitude of secondary reactions and the poor stability of the catalytic activity (inhibition of the catalyst by deposition of unsaturated oligomers) can be cited; these catalysts therefore have to be frequently regenerated. Moreover, the weak acidity of certain acid solids, such as molecular sieves, for example, necessitates the use of high reaction temperatures, which is prejudicial to obtaining hydrocarbons with a high octane rating.
European patent application EP-A-0539277 describes a catalyst containing silica and a solid acid phase comprising sulphuric acid; the silica according to said patent application is such that its porous volume is between 0.005 and 1.5 cm.sup.3 per gram and its specific surface is between 0.01 and 1,500 m.sup.2 per gram. Said acid phase can if required contain an additive selected from the group formed by H.sub.3 PO.sub.4, B(OH).sub.3, BF.sub.4 H, FSO.sub.3 H, CF.sub.3 CO.sub.2 H, SbF.sub.5, CF.sub.3 SO.sub.3 H and SO.sub.3.
French patent application FR-A-2 687 935 describes a catalyst containing a silica with a specific surface of between 0.01 and 1,500 m.sup.2 per gram and a total porous volume of between 0.005 and 1.5 cm.sup.3 per gram and an acid phase in solid state containing at least sulphuric acid and sulphuric anhydride, the silica having been impregnated with said acid phase. Said acid phase may possibly contain boric acid B(OH).sub.3, the content by weight of boric acid being between 0.01% and 50%.